I followed those instructions, and they seem to have worked. After running "service rocketchat start" and waiting a minute or so, I was able to access the installation. After rebooting the VM, Rocketchat started automatically and I was able to access it.
I did note an oddity on restarting, though. mongod took nearly 10 minutes to start. The proxmox console showed a sustained disk read of about 5M (though it isn't labeled with units--whether that's Mbits/sec, Mbytes/min, or Mwords/fortnight isn't clear) during that time, at the end of which there was a write spike of ~38M. I have no idea if this has anything to do with the packages installed--it could be my VM provisioning, my VM host's hardware, or any of a number of other things.